Two stage Gifford-McMahon cycle cryocoolers or refrigerators (hereafter GM cryocoolers or refrigerators) are devices that can be used to reach extremely low temperatures, such as approximately 4K, without a liquid refrigerant. A two stage GM cryocooler or refrigerator is discussed in U.S. Pat. No. 5,186,765.
In one application, two stage GM cryocoolers or refrigerators are used to cool the liquid helium (He) in magnetic resonance imaging (MRI) equipment in use as a medical diagnostic tool. For example, a GM cryocooler or refrigerator is used to cool the liquid He in MRI units so that the vapor pressure is lowered to reduce He losses and the need to frequently replace the liquid He in the unit. For example, without a GM cryocooler or refrigerator, the liquid He in an MRI unit typically needs to be replaced once every month. However, with a GM cryocooler to reduce He losses, the liquid He in the MRI unit needs to be replaced only once every six months, thereby allowing greater use of the MRI unit for medical diagnostic testing and lowering the overall cost of providing such medical diagnostic testing.
The potential use of lanthanide materials, which exhibit low magnetic ordering temperatures (e.g. less than 10K), as cryogenic regenerator materials (refrigerant or cold accumulating materials) was pointed out nearly 20 years ago by Buschow et al. in an article entitled "Extremely Large Heat Capacities between 4 and 10K", Cryogenics, vol. 15, (1975), pages 261-264. However, a practical lanthanide regenerator material was not developed and put into use until about 15 years later when use of Er.sub.3 Ni as a low temperature stage regenerator material in a two-stage GM cryocooler was proposed by Sahashi et al. in "New Magnetic Material R3T System with Extremely Large Heat Capacities Used as Heat Regenerators", Adv. Cryogenic Eng., vol. 35, (1990), pages 1175-1182 and by Kuriyama et al. in "High Efficient Two-Stage GM Refrigerator with Magnetic Material in Liquid Helium Temperature Region", Adv. Cryogenic Eng., vol. 35, (1990), pages 1261-1269.
These articles proposed the replacement of the prototype lead lower stage regenerator material with Er.sub.3 Ni material. Use of lead (Pb) as the prototype lower stage regenerator material permitted cooling to 10K, which was the lower temperature limit for a cryocooler using a lead lower stage regenerator material. Replacement of the lead lower stage regenerator material with Er.sub.3 Ni material permitted improved cooling to approximately 4.2K instead of the approximately 10K achievable with the previously used lead lower stage regenerator material with a reasonable refrigeration capacity at the lowest temperature. This improvement in cooling (i.e. to approximately 4.2K that is degrees Kelvin) is attributable to the significantly higher heat capacity of Er.sub.3 Ni than lead below 10K (the heat capacity of lead drops to zero at approximately 5K).
Although GM cryocoolers employing Er.sub.3 Ni lower temperature stage regenerators have proved effective, there are some cooling applications (such as real time magnetic resonance imaging (MRI units) also referred to recently as magnetic resonance therapy (MRT), superconductors, scientific instruments, spectroscopy, materials characterization, radio astronomy, matrix isolation, etc.) where it would be desirable to employ a lower temperature stage regenerator material having a higher heat capacity than Er.sub.3 Ni at about 10-15K and also for other applications (such as IR detectors, liquifaction of helium, MRI units, cryopumping, low noise measuring instruments, materials characterization, superconductors, radio astronomy, etc.) where it would be desirable to employ a lower temperature regenerator material having a higher heat capacity than Er.sub.3 Ni below about 5K.
It is an object of the invention to provide a low temperature passive magnetic heat regenerator (also known as magnetic refrigerant or cold accumulator) especially useful for the low temperature stage of a two stage GM cryocooler, as well as GM cryocooler using same, having significantly higher heat capacity than previously used lower temperature stage materials, such as Er.sub.3 Ni and Pb, between about 8 and about 19K.